In cutting and polishing silicon wafers, there form fine solid particles, usually 1 .mu.m and below, in the backgliding, dicing, and slicing processes. They mingle in washing water, and are discharged as they are. In view of recovering valuable silicon material, recycling or treating waste waters, it is of course necessary to separate them from water.
Hitherto, as for the separation of fine solid particles from waste solution, two systems are well known, and have been adopted: one a batch system, which comprises: feeding a certain amount of solution to a separator equipped with selective permeable membrane batchwise to separate water and solid particles by using the permeable action of the membrane, and feeding back the solution thus becoming concentrated with solid particles to feed again to the separator, and the other a continuous system, which additionally includes discharging part of solution concentrated with particles in a continuous manner.
According to such liquid-solid particle separation process employing only selective permeable membrane, it is certainly possible to have good working efficiency in the early stage by increasing the flux; it is because the solution is not still concentrated with particles. But as the concentration of particles reaches high level (usually 10,000 ppm or more) in solution, particles begin to stick to the selective permeable membrane so rapidly that they soon develop into aggregation there and eventually choke it as much as deteriorating the flowability of the solution. In consequence, not only does the separating capacity of the membrane decline but also there increasingly thrives a micro-organism on it.
In the meantime, other than the separation process using selective permeable membrane, the centrifugal process is well known, which is usually used to separate suspending 1-10 .mu.m particles from solution. If, however, this process would be applied to the separation of particles of submicron orders, it is difficult to have good result from it, because those particles have so too large specific surface area in proportion to their weight that they hardly precipitate due to the resistance with liquid (water), although it is effective when they tend to aggregate.
As stated above, as far as solution containing fine particles of submicron orders is concerned, the separation process by means of membrane is very effective in the early stage of separation where the particle aggregation is not so remarkable, contrary to the centrifugal separation, which is effective in the separation of aggregated particles.
Like this, the separation process by means of membrane and the separation process by means of centrifuge are contrary to each other in effectiveness according to the size of particles concerned.